Just in case you didn’t notice the crescentic lucency in the left lung base, the left costophrenic sulcus is wider and deeper than normal.

ie: Left subpulmonic pneumothorax with deep sulcus sign

reference: http://radiopaedia.org/articles/deep-sulcus-sign or http://www.learningradiology.com/notes/chestnotes/deepsulcussign.htm